In the dynamic industry of circuit design, the goal is always to develop components that are smaller and more efficient. The basic module of all digital circuits is the logic gate which performs according to simple Boolean expressions. Unlike analog circuits, digital circuits have only a few discrete input states. A Boolean variable typically has one of two values, signified as 1 or 0, and often referred to as TRUE or FALSE, respectively. In an electronic system these logic states may indicate that either a voltage is present to represent a value of 1, or no voltage is present to represent a value of 0. A gate's operation may be described in mathematical terms and Boolean algebra may be used for analyzing a circuit having gates. A gate is a simple digital circuit that produces a specific, predictable output condition based solely upon each possible combination of input conditions; hence logic implemented by gates is also referred to as combinational logic. Other than combinational logic, there is sequential logic wherein the output of a functional block also depends not only upon the inputs at a given time but also upon the prior states of the outputs of that functional block. When gates are connected to other gates, they are able to perform complex logical and arithmetic operations.
The simplest of logic functions are the Switch and the Inverter. A Switch has two states, being either Closed or Open; in other words, the input and output of a Switch are either connected to each other or not connected. In a two-state system of logic, an Inverter serves to change the state, namely, to invert 1's to 0's, and vice versa. These devices may be considered as degenerate logic elements, but they have their places.
Perhaps the most basic of gated logic elements is the OR-gate as described in U.S. patent application Ser. No. 10/719,119, which may be referred to here as the '119 application. It should be noted here that the term “OR” without a qualifier is generally taken to mean Inclusive-OR. In its most basic form, this type of OR-gate accomplishes the function of disjunction, that is, it outputs a value of 1 if any one or more of its inputs has a value of 1. The Inclusive-OR gate will have an output value of 0 only if all of the values at its input are 0. When the output state of an OR-gate is inverted, the combined result is a NOR-gate, which has been addressed in U.S. patent application Ser. No. 11/273,362.
A second of the most common gates in digital logic design is the AND gate which performs the Boolean operation of conjunction. The AND gate outputs a value of 1 only if all of its input values are 1. If any one or more of the inputs to an AND-gate have a value of 0 then its output will be 0. An AND-gate which has had its output state inverted is referred to as a NAND-gate.
Of the logic elements having only two inputs, the remaining one is the Exclusive-OR gate, commonly referred to as an XOR-gate. The XOR-gate assumes an output state of 1 if only one of its inputs is 1, and the other is a 0. The inverted form of the XOR-gate is called an XNOR-gate (or alternately, an XAND gate) and will have a 1 at its output for either of the conditions wherein both inputs are the same, that is, both inputs are 1 or both are 0. The XOR function is necessary to accomplish the arithmetic operation of addition, which is a precursor to an arithmetic multiplier.
Each of these two-input logic functions may be implemented with gates having relatively few components. For example, the function of an OR-gate may be accomplished by two diodes in parallel or by two transistors in parallel. In the transistor example, when a gate voltage is present at the gate of one or both of the transistors a 1 value will result at the output of the OR-gate. An AND-gate may be constructed with two transistors in series, where a voltage is required at the gates of both transistors in order to produce a value of 1 at the output of the AND-gate. Implementation of the XOR function is a bit more complex. As can be expected, a vast number of OR-gates, AND-gates and other logic elements are required to construct the complex circuitry of a computer system. Because of this, even slight reductions in the size of a single logic gate would result in significant improvements to the overall architecture of a computer system.
It would be an advancement in the art to provide an AND-gate, an OR-gate, an XOR-gate, or any other logic circuit element that is smaller, more efficient, and has superior gate voltage control. Such devices are disclosed and claimed herein.